The present invention pertains to an apparatus for conveying preforms to a machine for the production of containers from a thermoplastic material, wherein the containers are formed by the action of a blowing pressure.
In the process of forming containers by the action of blowing pressure, preforms of a thermoplastic material such as preforms of PET (polyethylene terephthalate) are sent to various processing stations within a blow-molding machine. A blow-molding machine of this type typically comprises a heating device and a blowing device, in the area of which the previously tempered preform is expanded into a container by biaxial orientation. The expansion occurs by means of compressed air, which is introduced into the preform to be expanded. The course of the processing technology involved in a preform expansion of this type is explained in DE-OS 43 40 291. The basic configuration of a blow-molding station for forming containers is described in DE-OS 42 12 583. Possibilities for tempering the preforms are explained in DE-OS 23 52 926.
There are two alternative ways in which the preforms can be sent to the blow-molding machine. In the case of the so-called “one-step” method, the preforms are produced by an injection-molding process, and, immediately after they have been injection-molded, they are transported directly to the blow-molding machine by a conveying apparatus. In the case of the so-called “two-step” method, the preforms are first produced by an injection-molding process and then stored; they are conditioned with respect to their temperature and then blown into containers only at a later time. The production of the preforms and the blow-molding of the preforms into containers can take place at different times and at different places.
Both methods have in common that the preforms are sent to the blow-molding machine by a conveying apparatus. Known apparatuses consist of, for example, a vertical conveyor, by which the preforms, which are stored in a collecting bin, are transported to a conveying device. By means of the conveying device, usually a conveyor belt, the preforms are transported to a sorting device. This sorting device has the task of orienting the preforms into a defined position so that they can be transferred to, for example, a downline blow-molding machine. Known sorting devices (so-called roll sorters) consist, for example, of two conveying rolls, which rotate in opposite directions and between which a small gap remains, which is selected in such a way that the preforms can be held, suspended by their collars, between the rotating conveying rolls. The conveying rolls are arranged at an angle, so that the preforms slide down the incline, i.e., in the conveying direction, along the conveying rolls. The collar of the preform moves in the conveying direction along the preconfigured guide surfaces of the conveying rolls, the contour of these guide surfaces being in the form of part of a circle. “Contour” means here the form of the guide surface in the cross-sectional plane perpendicular to the conveying direction. Depending on the radius of the collar, the preform travels along the guide surface at a different level. The preforms are sorted, suspended between the rotating rolls, and transported in the conveying direction to a gravity conveying device, which follows the sorting device, and which also comprises a guide surface with a guide surface contour of a certain geometry, along which the sorted preforms are transported to the blow-molding machine.
There are essentially two different standard types of gravity conveying devices. First, a so-called “gravity rail” is frequently used, which is formed out of two parallel rails in such a way that a gap remains between them, in which the preforms can be transported in suspended fashion, wherein the gravity rail is configured in such a way that it comprises an incline, so that the preforms slide along the gravity rail by the force of gravity. The guide surface of the gravity rail can be described in a good approximation as a slanted plane; in this case the contour of the guide surfaces is then a straight horizontal line. The use of an air conveying section is also known as a gravity conveying device. Here, too, the preforms are conveyed in suspended fashion. Air conveying sections are usually arranged horizontally, and the preforms are conveyed by the action of streams of air, which accelerate the preforms. An example of an air conveying section of this type is described in WO 2011/138321 A1. Here too, the guide surface, along which the collars of the preforms are guided, is essentially a flat surface. Additional gravity conveying devices are also known from the prior art, however.
In the case of the known apparatuses, problems frequently occur in the area where the preforms are transferred from the sorting device to the gravity conveying device. The preforms are transferred from the sorting device to the gravity conveying device by way of a transfer device. A transfer device of this type known from the prior art is configured as a separate connecting element, for example, which is arranged between the sorting device and the gravity conveying device and is connected to both. At the gravity conveyor end, the connecting element comprises a guide surface contour, which adjoins the guide surface contour of the gravity conveying device.
When the preforms are being transferred, the preforms drop from the sorting device onto this stationary connecting element.
Depending on the type of container being produced, the geometry of the preforms can change; as a result, the preforms will follow a different path along the guide surface of the rolls of the roll conveyor, e.g., a path situated at a higher level. As a rule, it is therefore also necessary to adapt the connecting element to this changed situation. This suffers from the disadvantage that, each time there is a changeover from one type of preform to another, the connecting element must be readjusted to suit the type of preform in question, a process which is always time-consuming. The connecting element is also located in an area which is difficult to reach. The connecting element of known apparatuses, furthermore, is configured in such a way that the preforms drop several millimeters downward onto the connecting element. This frequently leads to a tipping or jamming of the preforms on the connecting element and, in association with that, to an interruption in the supply of preforms and in turn to the stoppage of the operation of the blow-molding machine.